Intake Air By-Pass (IAB) Control System (Prelude)

Intake air by-pass (IAB) control solenoid valve is located at lower rear of engine. See COMPONENT LOCATIONS . IAB control solenoid valve is off when engine speed is above 4900 RPM.

1. Start engine and allow to idle. Disconnect vacuum hose from IAB control diaphragm and connect a vacuum pump/gauge to hose. See Fig 1. If vacuum does not exist, go to next step. If vacuum exists, go to step  6.
   Fig 1: Locating Intake Air By-Pass Control Diaphragm (Prelude)

   Courtesy of AMERICAN HONDA MOTOR CO., INC.
2. Remove vacuum pump/gauge and reconnect vacuum hose at IAB control diaphragm. Disconnect vacuum hose from IAB vacuum tank and connect vacuum pump/gauge to IAB vacuum tank. See VACUUM DIAGRAMS article. If vacuum exists at IAB vacuum tank, go to next step. If vacuum does not exist at IAB vacuum tank, repair blockage or vacuum leak between IAB vacuum tank and intake manifold.
3. Turn ignition switch off. Disconnect IAB control solenoid valve 2-pin connector. Turn ignition on. Measure voltage between IAB control solenoid valve connector terminals (Black/Yellow wire and Red/Blue wire). If battery voltage does not exist, go to next step. If battery voltage exists, replace IAB control solenoid valve.
4. Measure voltage between body ground and IAB control solenoid valve 2-pin connector terminal No. 1 (Black/Yellow wire). If battery voltage exists, go to next step. If battery voltage does not exist, repair open in Black/Yellow wire between IAB control solenoid valve connector and No. 14 (15-amp) fuse located in underdash fuse/relay box. See WIRING DIAGRAMS article.
5. Turn ignition off. Reconnect IAB control solenoid valve 2-pin connector. Turn ignition on. Backprobing, connect a jumper wire between PCM connector "A" (32-pin connector) terminals No. 9 (Brown/Black wire) and No. 26 (Red/Blue wire). See Fig 2. If IAB control solenoid valve clicks when jumper wire is connected, substitute a known-good PCM and retest. See SUBSTITUTING POWERTRAIN CONTROL MODULE under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - PRELUDE article. If symptom or indication goes away, replace original PCM. If IAB control solenoid valve does not click when jumper wire is connected, repair open in Red/Blue wire between IAB control solenoid valve and PCM connectors. See WIRING DIAGRAMS article.
   Fig 2: Identifying PCM Connector "A" (32-Pin Connector) Terminals (Accord, CR-V, Insight, Prelude & S2000)

   Courtesy of AMERICAN HONDA MOTOR CO., INC.
6. Increase engine speed to above 5000 RPM. Check for vacuum at disconnected IAB control diaphragm hose. See Fig 1. If vacuum does not exist, IAB control system is okay. If vacuum exists, go to next step.
7. Turn ignition off. Disconnect IAB control solenoid valve 2-pin connector. Check for vacuum again at disconnected IAB control diaphragm hose. If vacuum does not exist, go to next step. If vacuum exists, replace IAB control solenoid valve.
8. Turn ignition off. Disconnect PCM connector "A" (32-pin connector). Check for continuity between body ground and IAB control solenoid valve 2-pin connector terminal No. 2 (Red/Blue wire). If continuity exists, repair short in Red/Blue wire between IAB control solenoid valve and PCM connectors. See WIRING DIAGRAMS article. If continuity does not exist, substitute a known-good PCM and retest. See SUBSTITUTING POWERTRAIN CONTROL MODULE under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - PRELUDE article. If symptom or indication goes away, replace original PCM.